In recent years, the water treatment using a membrane filter excellent in completeness of separation and compactness has attracted attention since interest in environmental problems has increased and also the regulations on water quality have been tightened. As the membrane filter, for example, a hollow porous membrane is used and employed in a variety of water treatment such as the sewage and wastewater treatment or the water purification treatment.
Hollow porous membranes with various configurations have been investigated. For example, there is a hollow porous membrane in which a braid support formed by circular knitting the yarn into a cylindrical shape is used as a support for a hollow porous membrane (hereinafter, simply referred to as the support in some cases) and a porous membrane layer is provided on the outer circumferential surface thereof. The hollow porous membrane is produced as follows, for example. First, a membrane forming solution is discharged from a double pipe spinning nozzle when the support continuously passes through the double pipe spinning nozzle so that the membrane forming solution is applied to the outer circumferential surface of the support. Thereafter, the support coated with the membrane forming solution is allowed to pass through a coagulation bath so that the membrane forming solution is coagulated by a coagulating liquid in the coagulation bath. The hollow porous membrane is obtained in this manner.
The hollow porous membrane is required to exhibit excellent separation properties and permeability or high mechanical properties. For example, in the water treatment field, it is required to physically remove the virus having a particle size of about from tens to 50 nm contained in the river or the like because of tightened regulations and the like.
In addition, in a case in which the hollow porous membrane is used together with the activated carbon in various kinds of water treatment or a case in which inorganic particles such as sand are contained in the water to be treated, the filtration performance of the membrane decreases in some cases since the membrane surface comes in contact with the activated carbon or the inorganic particles at the time of the air scrubbing process or the like so that the membrane surface is worn or the activated carbon or the inorganic particles pierce into the membrane surface so as to generate a defective point on the membrane. Hence, the hollow porous membrane is required to has the resistance to such piercing or wear and thus to be able to maintain the virus removal performance even at the time of being used together with the activated carbon or in a case in which inorganic particles are contained in the water to be treated. As the virus removal mechanism, generally, there are sieving (screening) filtration in which the filtration is performed by a sieve having a pore size smaller than the virus, so-called depth filtration in which the virus is trapped in micropores, and the like.
As the hollow porous membrane, for example, a membrane is described in Patent Document 1 in which a separating functional layer which has a three-dimensional network structure and contains polyvinylidene fluoride having a melt viscosity of 2500 Pa·s or more is laminated on a polyvinylidene fluoride-based support layer having a spherical structure. It is described in Patent Document 1 that the membrane exhibits high virus removal performance and physical strength.
In addition, a hollow porous membrane is disclosed in Patent Document 2 which has an outer layer composed of a thermoplastic resin having a mass average molecular weight of 400,000 or more and a soft inner layer composed of a thermoplastic resin having a mass average molecular weight of less than 400,000. It is described in Patent Document 2 that, since the hollow porous membrane has a soft inner layer, the pressing force on the outer surface is dispersed and lowered when the membranes come in contact with each other and thus the abrasion can be diminished.
In addition, a hollow porous membrane is disclosed in Patent Document 3 which has a compact layer on the membrane surface and inside the membrane. It is described in Patent Document 3 that this hollow porous membrane can maintain significantly high safety since the compact layer inside the membrane blocks the removal target substance even if the compact layer on the membrane surface is damaged and thus the removal target substance blocking rate thereof is lowered.